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        <h1>Node.js v5.6.0 Documentation</h1>
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<li><a href="modules.html#modules_modules">Modules</a><ul>
<li><a href="modules.html#modules_accessing_the_main_module">Accessing the main module</a></li>
<li><a href="modules.html#modules_addenda_package_manager_tips">Addenda: Package Manager Tips</a></li>
<li><a href="modules.html#modules_all_together">All Together...</a></li>
<li><a href="modules.html#modules_caching">Caching</a><ul>
<li><a href="modules.html#modules_module_caching_caveats">Module Caching Caveats</a></li>
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<li><a href="modules.html#modules_core_modules">Core Modules</a></li>
<li><a href="modules.html#modules_cycles">Cycles</a></li>
<li><a href="modules.html#modules_file_modules">File Modules</a></li>
<li><a href="modules.html#modules_folders_as_modules">Folders as Modules</a></li>
<li><a href="modules.html#modules_loading_from_node_modules_folders">Loading from <code>node_modules</code> Folders</a></li>
<li><a href="modules.html#modules_loading_from_the_global_folders">Loading from the global folders</a></li>
<li><a href="modules.html#modules_the_module_object">The <code>module</code> Object</a><ul>
<li><a href="modules.html#modules_module_children">module.children</a></li>
<li><a href="modules.html#modules_module_exports">module.exports</a><ul>
<li><a href="modules.html#modules_exports_alias">exports alias</a></li>
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      <div id="apicontent">
        <h1>Modules<span><a class="mark" href="modules.html#modules_modules" id="modules_modules">#</a></span></h1>
<pre class="api_stability_3">Stability: 3 - Locked</pre><!--name=module-->

<p>Node.js has a simple module loading system.  In Node.js, files and modules are
in one-to-one correspondence.  As an example, <code>foo.js</code> loads the module
<code>circle.js</code> in the same directory.

</p>
<p>The contents of <code>foo.js</code>:

</p>
<pre><code class="js">const circle = require(&#39;./circle.js&#39;);
console.log( `The area of a circle of radius 4 is ${circle.area(4)}`);</code></pre>
<p>The contents of <code>circle.js</code>:

</p>
<pre><code class="js">const PI = Math.PI;

exports.area = (r) =&gt; PI * r * r;

exports.circumference = (r) =&gt; 2 * PI * r;</code></pre>
<p>The module <code>circle.js</code> has exported the functions <code>area()</code> and
<code>circumference()</code>.  To add functions and objects to the root of your module,
you can add them to the special <code>exports</code> object.

</p>
<p>Variables local to the module will be private, as though the module was wrapped
in a function. In this example the variable <code>PI</code> is private to <code>circle.js</code>.

</p>
<p>If you want the root of your module&#39;s export to be a function (such as a
constructor) or if you want to export a complete object in one assignment
instead of building it one property at a time, assign it to <code>module.exports</code>
instead of <code>exports</code>.

</p>
<p>Below, <code>bar.js</code> makes use of the <code>square</code> module, which exports a constructor:

</p>
<pre><code class="js">const square = require(&#39;./square.js&#39;);
var mySquare = square(2);
console.log(`The area of my square is ${mySquare.area()}`);</code></pre>
<p>The <code>square</code> module is defined in <code>square.js</code>:

</p>
<pre><code class="js">// assigning to exports will not modify module, must use module.exports
module.exports = (width) =&gt; {
  return {
    area: () =&gt; width * width
  };
}</code></pre>
<p>The module system is implemented in the <code>require(&quot;module&quot;)</code> module.

</p>
<h2>Accessing the main module<span><a class="mark" href="modules.html#modules_accessing_the_main_module" id="modules_accessing_the_main_module">#</a></span></h2>
<!-- type=misc -->

<p>When a file is run directly from Node.js, <code>require.main</code> is set to its
<code>module</code>. That means that you can determine whether a file has been run
directly by testing

</p>
<pre><code class="js">require.main === module</code></pre>
<p>For a file <code>foo.js</code>, this will be <code>true</code> if run via <code>node foo.js</code>, but
<code>false</code> if run by <code>require(&#39;./foo&#39;)</code>.

</p>
<p>Because <code>module</code> provides a <code>filename</code> property (normally equivalent to
<code>__filename</code>), the entry point of the current application can be obtained
by checking <code>require.main.filename</code>.

</p>
<h2>Addenda: Package Manager Tips<span><a class="mark" href="modules.html#modules_addenda_package_manager_tips" id="modules_addenda_package_manager_tips">#</a></span></h2>
<!-- type=misc -->

<p>The semantics of Node.js&#39;s <code>require()</code> function were designed to be general
enough to support a number of reasonable directory structures. Package manager
programs such as <code>dpkg</code>, <code>rpm</code>, and <code>npm</code> will hopefully find it possible to
build native packages from Node.js modules without modification.

</p>
<p>Below we give a suggested directory structure that could work:

</p>
<p>Let&#39;s say that we wanted to have the folder at
<code>/usr/lib/node/&lt;some-package&gt;/&lt;some-version&gt;</code> hold the contents of a
specific version of a package.

</p>
<p>Packages can depend on one another. In order to install package <code>foo</code>, you
may have to install a specific version of package <code>bar</code>.  The <code>bar</code> package
may itself have dependencies, and in some cases, these dependencies may even
collide or form cycles.

</p>
<p>Since Node.js looks up the <code>realpath</code> of any modules it loads (that is,
resolves symlinks), and then looks for their dependencies in the <code>node_modules</code>
folders as described <a href="modules.html#modules_loading_from_node_modules_folders">here</a>, this
situation is very simple to resolve with the following architecture:

</p>
<ul>
<li><code>/usr/lib/node/foo/1.2.3/</code> - Contents of the <code>foo</code> package, version 1.2.3.</li>
<li><code>/usr/lib/node/bar/4.3.2/</code> - Contents of the <code>bar</code> package that <code>foo</code>
depends on.</li>
<li><code>/usr/lib/node/foo/1.2.3/node_modules/bar</code> - Symbolic link to
<code>/usr/lib/node/bar/4.3.2/</code>.</li>
<li><code>/usr/lib/node/bar/4.3.2/node_modules/*</code> - Symbolic links to the packages
that <code>bar</code> depends on.</li>
</ul>
<p>Thus, even if a cycle is encountered, or if there are dependency
conflicts, every module will be able to get a version of its dependency
that it can use.

</p>
<p>When the code in the <code>foo</code> package does <code>require(&#39;bar&#39;)</code>, it will get the
version that is symlinked into <code>/usr/lib/node/foo/1.2.3/node_modules/bar</code>.
Then, when the code in the <code>bar</code> package calls <code>require(&#39;quux&#39;)</code>, it&#39;ll get
the version that is symlinked into
<code>/usr/lib/node/bar/4.3.2/node_modules/quux</code>.

</p>
<p>Furthermore, to make the module lookup process even more optimal, rather
than putting packages directly in <code>/usr/lib/node</code>, we could put them in
<code>/usr/lib/node_modules/&lt;name&gt;/&lt;version&gt;</code>.  Then Node.js will not bother
looking for missing dependencies in <code>/usr/node_modules</code> or <code>/node_modules</code>.

</p>
<p>In order to make modules available to the Node.js REPL, it might be useful to
also add the <code>/usr/lib/node_modules</code> folder to the <code>$NODE_PATH</code> environment
variable.  Since the module lookups using <code>node_modules</code> folders are all
relative, and based on the real path of the files making the calls to
<code>require()</code>, the packages themselves can be anywhere.

</p>
<h2>All Together...<span><a class="mark" href="modules.html#modules_all_together" id="modules_all_together">#</a></span></h2>
<!-- type=misc -->

<p>To get the exact filename that will be loaded when <code>require()</code> is called, use
the <code>require.resolve()</code> function.

</p>
<p>Putting together all of the above, here is the high-level algorithm
in pseudocode of what require.resolve does:

</p>
<pre><code>require(X) from module at path Y
1. If X is a core module,
   a. return the core module
   b. STOP
2. If X begins with &#39;./&#39; or &#39;/&#39; or &#39;../&#39;
   a. LOAD_AS_FILE(Y + X)
   b. LOAD_AS_DIRECTORY(Y + X)
3. LOAD_NODE_MODULES(X, dirname(Y))
4. THROW &quot;not found&quot;

LOAD_AS_FILE(X)
1. If X is a file, load X as JavaScript text.  STOP
2. If X.js is a file, load X.js as JavaScript text.  STOP
3. If X.json is a file, parse X.json to a JavaScript Object.  STOP
4. If X.node is a file, load X.node as binary addon.  STOP

LOAD_AS_DIRECTORY(X)
1. If X/package.json is a file,
   a. Parse X/package.json, and look for &quot;main&quot; field.
   b. let M = X + (json main field)
   c. LOAD_AS_FILE(M)
2. If X/index.js is a file, load X/index.js as JavaScript text.  STOP
3. If X/index.json is a file, parse X/index.json to a JavaScript object. STOP
4. If X/index.node is a file, load X/index.node as binary addon.  STOP

LOAD_NODE_MODULES(X, START)
1. let DIRS=NODE_MODULES_PATHS(START)
2. for each DIR in DIRS:
   a. LOAD_AS_FILE(DIR/X)
   b. LOAD_AS_DIRECTORY(DIR/X)

NODE_MODULES_PATHS(START)
1. let PARTS = path split(START)
2. let I = count of PARTS - 1
3. let DIRS = []
4. while I &gt;= 0,
   a. if PARTS[I] = &quot;node_modules&quot; CONTINUE
   c. DIR = path join(PARTS[0 .. I] + &quot;node_modules&quot;)
   b. DIRS = DIRS + DIR
   c. let I = I - 1
5. return DIRS</code></pre>
<h2>Caching<span><a class="mark" href="modules.html#modules_caching" id="modules_caching">#</a></span></h2>
<!--type=misc-->

<p>Modules are cached after the first time they are loaded.  This means
(among other things) that every call to <code>require(&#39;foo&#39;)</code> will get
exactly the same object returned, if it would resolve to the same file.

</p>
<p>Multiple calls to <code>require(&#39;foo&#39;)</code> may not cause the module code to be
executed multiple times.  This is an important feature.  With it,
&quot;partially done&quot; objects can be returned, thus allowing transitive
dependencies to be loaded even when they would cause cycles.

</p>
<p>If you want to have a module execute code multiple times, then export a
function, and call that function.

</p>
<h3>Module Caching Caveats<span><a class="mark" href="modules.html#modules_module_caching_caveats" id="modules_module_caching_caveats">#</a></span></h3>
<!--type=misc-->

<p>Modules are cached based on their resolved filename.  Since modules may
resolve to a different filename based on the location of the calling
module (loading from <code>node_modules</code> folders), it is not a <em>guarantee</em>
that <code>require(&#39;foo&#39;)</code> will always return the exact same object, if it
would resolve to different files.

</p>
<h2>Core Modules<span><a class="mark" href="modules.html#modules_core_modules" id="modules_core_modules">#</a></span></h2>
<!--type=misc-->

<p>Node.js has several modules compiled into the binary.  These modules are
described in greater detail elsewhere in this documentation.

</p>
<p>The core modules are defined within Node.js&#39;s source and are located in the
<code>lib/</code> folder.

</p>
<p>Core modules are always preferentially loaded if their identifier is
passed to <code>require()</code>.  For instance, <code>require(&#39;http&#39;)</code> will always
return the built in HTTP module, even if there is a file by that name.

</p>
<h2>Cycles<span><a class="mark" href="modules.html#modules_cycles" id="modules_cycles">#</a></span></h2>
<!--type=misc-->

<p>When there are circular <code>require()</code> calls, a module might not have finished
executing when it is returned.

</p>
<p>Consider this situation:

</p>
<p><code>a.js</code>:

</p>
<pre><code>console.log(&#39;a starting&#39;);
exports.done = false;
const b = require(&#39;./b.js&#39;);
console.log(&#39;in a, b.done = %j&#39;, b.done);
exports.done = true;
console.log(&#39;a done&#39;);</code></pre>
<p><code>b.js</code>:

</p>
<pre><code>console.log(&#39;b starting&#39;);
exports.done = false;
const a = require(&#39;./a.js&#39;);
console.log(&#39;in b, a.done = %j&#39;, a.done);
exports.done = true;
console.log(&#39;b done&#39;);</code></pre>
<p><code>main.js</code>:

</p>
<pre><code>console.log(&#39;main starting&#39;);
const a = require(&#39;./a.js&#39;);
const b = require(&#39;./b.js&#39;);
console.log(&#39;in main, a.done=%j, b.done=%j&#39;, a.done, b.done);</code></pre>
<p>When <code>main.js</code> loads <code>a.js</code>, then <code>a.js</code> in turn loads <code>b.js</code>.  At that
point, <code>b.js</code> tries to load <code>a.js</code>.  In order to prevent an infinite
loop, an <strong>unfinished copy</strong> of the <code>a.js</code> exports object is returned to the
<code>b.js</code> module.  <code>b.js</code> then finishes loading, and its <code>exports</code> object is
provided to the <code>a.js</code> module.

</p>
<p>By the time <code>main.js</code> has loaded both modules, they&#39;re both finished.
The output of this program would thus be:

</p>
<pre><code>$ node main.js
main starting
a starting
b starting
in b, a.done = false
b done
in a, b.done = true
a done
in main, a.done=true, b.done=true</code></pre>
<p>If you have cyclic module dependencies in your program, make sure to
plan accordingly.

</p>
<h2>File Modules<span><a class="mark" href="modules.html#modules_file_modules" id="modules_file_modules">#</a></span></h2>
<!--type=misc-->

<p>If the exact filename is not found, then Node.js will attempt to load the
required filename with the added extensions: <code>.js</code>, <code>.json</code>, and finally
<code>.node</code>.

</p>
<p><code>.js</code> files are interpreted as JavaScript text files, and <code>.json</code> files are
parsed as JSON text files. <code>.node</code> files are interpreted as compiled addon
modules loaded with <code>dlopen</code>.

</p>
<p>A required module prefixed with <code>&#39;/&#39;</code> is an absolute path to the file.  For
example, <code>require(&#39;/home/marco/foo.js&#39;)</code> will load the file at
<code>/home/marco/foo.js</code>.

</p>
<p>A required module prefixed with <code>&#39;./&#39;</code> is relative to the file calling
<code>require()</code>. That is, <code>circle.js</code> must be in the same directory as <code>foo.js</code> for
<code>require(&#39;./circle&#39;)</code> to find it.

</p>
<p>Without a leading &#39;/&#39;, &#39;./&#39;, or &#39;../&#39; to indicate a file, the module must
either be a core module or is loaded from a <code>node_modules</code> folder.

</p>
<p>If the given path does not exist, <code>require()</code> will throw an <a href="errors.html#errors_class_error"><code>Error</code></a> with its
<code>code</code> property set to <code>&#39;MODULE_NOT_FOUND&#39;</code>.

</p>
<h2>Folders as Modules<span><a class="mark" href="modules.html#modules_folders_as_modules" id="modules_folders_as_modules">#</a></span></h2>
<!--type=misc-->

<p>It is convenient to organize programs and libraries into self-contained
directories, and then provide a single entry point to that library.
There are three ways in which a folder may be passed to <code>require()</code> as
an argument.

</p>
<p>The first is to create a <code>package.json</code> file in the root of the folder,
which specifies a <code>main</code> module.  An example package.json file might
look like this:

</p>
<pre><code>{ &quot;name&quot; : &quot;some-library&quot;,
  &quot;main&quot; : &quot;./lib/some-library.js&quot; }</code></pre>
<p>If this was in a folder at <code>./some-library</code>, then
<code>require(&#39;./some-library&#39;)</code> would attempt to load
<code>./some-library/lib/some-library.js</code>.

</p>
<p>This is the extent of Node.js&#39;s awareness of package.json files.

</p>
<p>If there is no package.json file present in the directory, then Node.js
will attempt to load an <code>index.js</code> or <code>index.node</code> file out of that
directory.  For example, if there was no package.json file in the above
example, then <code>require(&#39;./some-library&#39;)</code> would attempt to load:

</p>
<ul>
<li><code>./some-library/index.js</code></li>
<li><code>./some-library/index.node</code></li>
</ul>
<h2>Loading from <code>node_modules</code> Folders<span><a class="mark" href="modules.html#modules_loading_from_node_modules_folders" id="modules_loading_from_node_modules_folders">#</a></span></h2>
<!--type=misc-->

<p>If the module identifier passed to <code>require()</code> is not a native module,
and does not begin with <code>&#39;/&#39;</code>, <code>&#39;../&#39;</code>, or <code>&#39;./&#39;</code>, then Node.js starts at the
parent directory of the current module, and adds <code>/node_modules</code>, and
attempts to load the module from that location. Node will not append
<code>node_modules</code> to a path already ending in <code>node_modules</code>.

</p>
<p>If it is not found there, then it moves to the parent directory, and so
on, until the root of the file system is reached.

</p>
<p>For example, if the file at <code>&#39;/home/ry/projects/foo.js&#39;</code> called
<code>require(&#39;bar.js&#39;)</code>, then Node.js would look in the following locations, in
this order:

</p>
<ul>
<li><code>/home/ry/projects/node_modules/bar.js</code></li>
<li><code>/home/ry/node_modules/bar.js</code></li>
<li><code>/home/node_modules/bar.js</code></li>
<li><code>/node_modules/bar.js</code></li>
</ul>
<p>This allows programs to localize their dependencies, so that they do not
clash.

</p>
<p>You can require specific files or sub modules distributed with a module by
including a path suffix after the module name. For instance
<code>require(&#39;example-module/path/to/file&#39;)</code> would resolve <code>path/to/file</code>
relative to where <code>example-module</code> is located. The suffixed path follows the
same module resolution semantics.

</p>
<h2>Loading from the global folders<span><a class="mark" href="modules.html#modules_loading_from_the_global_folders" id="modules_loading_from_the_global_folders">#</a></span></h2>
<!-- type=misc -->

<p>If the <code>NODE_PATH</code> environment variable is set to a colon-delimited list
of absolute paths, then Node.js will search those paths for modules if they
are not found elsewhere.  (Note: On Windows, <code>NODE_PATH</code> is delimited by
semicolons instead of colons.)

</p>
<p><code>NODE_PATH</code> was originally created to support loading modules from
varying paths before the current <a href="modules.html#modules_all_together">module resolution</a> algorithm was frozen.

</p>
<p><code>NODE_PATH</code> is still supported, but is less necessary now that the Node.js
ecosystem has settled on a convention for locating dependent modules.
Sometimes deployments that rely on <code>NODE_PATH</code> show surprising behavior
when people are unaware that <code>NODE_PATH</code> must be set.  Sometimes a
module&#39;s dependencies change, causing a different version (or even a
different module) to be loaded as the <code>NODE_PATH</code> is searched.

</p>
<p>Additionally, Node.js will search in the following locations:

</p>
<ul>
<li>1: <code>$HOME/.node_modules</code></li>
<li>2: <code>$HOME/.node_libraries</code></li>
<li>3: <code>$PREFIX/lib/node</code></li>
</ul>
<p>Where <code>$HOME</code> is the user&#39;s home directory, and <code>$PREFIX</code> is Node.js&#39;s
configured <code>node_prefix</code>.

</p>
<p>These are mostly for historic reasons.  <strong>You are highly encouraged
to place your dependencies locally in <code>node_modules</code> folders.</strong>  They
will be loaded faster, and more reliably.

</p>
<h2>The <code>module</code> Object<span><a class="mark" href="modules.html#modules_the_module_object" id="modules_the_module_object">#</a></span></h2>
<!-- type=var -->
<!-- name=module -->

<ul>
<li>{Object}</li>
</ul>
<p>In each module, the <code>module</code> free variable is a reference to the object
representing the current module.  For convenience, <code>module.exports</code> is
also accessible via the <code>exports</code> module-global. <code>module</code> isn&#39;t actually
a global but rather local to each module.

</p>
<h3>module.children<span><a class="mark" href="modules.html#modules_module_children" id="modules_module_children">#</a></span></h3>
<div class="signature"><ul>
<li><span class="type">Array</span></li>
</div></ul>
<p>The module objects required by this one.

</p>
<h3>module.exports<span><a class="mark" href="modules.html#modules_module_exports" id="modules_module_exports">#</a></span></h3>
<div class="signature"><ul>
<li><span class="type">Object</span></li>
</div></ul>
<p>The <code>module.exports</code> object is created by the Module system. Sometimes this is
not acceptable; many want their module to be an instance of some class. To do
this, assign the desired export object to <code>module.exports</code>. Note that assigning
the desired object to <code>exports</code> will simply rebind the local <code>exports</code> variable,
which is probably not what you want to do.

</p>
<p>For example suppose we were making a module called <code>a.js</code>

</p>
<pre><code class="js">const EventEmitter = require(&#39;events&#39;);

module.exports = new EventEmitter();

// Do some work, and after some time emit
// the &#39;ready&#39; event from the module itself.
setTimeout(() =&gt; {
  module.exports.emit(&#39;ready&#39;);
}, 1000);</code></pre>
<p>Then in another file we could do

</p>
<pre><code class="js">const a = require(&#39;./a&#39;);
a.on(&#39;ready&#39;, () =&gt; {
  console.log(&#39;module a is ready&#39;);
});</code></pre>
<p>Note that assignment to <code>module.exports</code> must be done immediately. It cannot be
done in any callbacks.  This does not work:

</p>
<p>x.js:

</p>
<pre><code class="js">setTimeout(() =&gt; {
  module.exports = { a: &#39;hello&#39; };
}, 0);</code></pre>
<p>y.js:

</p>
<pre><code class="js">const x = require(&#39;./x&#39;);
console.log(x.a);</code></pre>
<h4>exports alias<span><a class="mark" href="modules.html#modules_exports_alias" id="modules_exports_alias">#</a></span></h4>
<p>The <code>exports</code> variable that is available within a module starts as a reference
to <code>module.exports</code>. As with any variable, if you assign a new value to it, it
is no longer bound to the previous value.

</p>
<p>To illustrate the behavior, imagine this hypothetical implementation of
<code>require()</code>:

</p>
<pre><code class="js">function require(...) {
  // ...
  ((module, exports) =&gt; {
    // Your module code here
    exports = some_func;        // re-assigns exports, exports is no longer
                                // a shortcut, and nothing is exported.
    module.exports = some_func; // makes your module export 0
  })(module, module.exports);
  return module;
}</code></pre>
<p>As a guideline, if the relationship between <code>exports</code> and <code>module.exports</code>
seems like magic to you, ignore <code>exports</code> and only use <code>module.exports</code>.

</p>
<h3>module.filename<span><a class="mark" href="modules.html#modules_module_filename" id="modules_module_filename">#</a></span></h3>
<div class="signature"><ul>
<li><span class="type">String</span></li>
</div></ul>
<p>The fully resolved filename to the module.

</p>
<h3>module.id<span><a class="mark" href="modules.html#modules_module_id" id="modules_module_id">#</a></span></h3>
<div class="signature"><ul>
<li><span class="type">String</span></li>
</div></ul>
<p>The identifier for the module.  Typically this is the fully resolved
filename.

</p>
<h3>module.loaded<span><a class="mark" href="modules.html#modules_module_loaded" id="modules_module_loaded">#</a></span></h3>
<div class="signature"><ul>
<li><span class="type">Boolean</span></li>
</div></ul>
<p>Whether or not the module is done loading, or is in the process of
loading.

</p>
<h3>module.parent<span><a class="mark" href="modules.html#modules_module_parent" id="modules_module_parent">#</a></span></h3>
<div class="signature"><ul>
<li><span class="type">Module Object</span></li>
</div></ul>
<p>The module that first required this one.

</p>
<h3>module.require(id)<span><a class="mark" href="modules.html#modules_module_require_id" id="modules_module_require_id">#</a></span></h3>
<div class="signature"><ul>
<li><code>id</code> <span class="type">String</span></li>
<li>Return: <span class="type">Object</span> <code>module.exports</code> from the resolved module</li>
</div></ul>
<p>The <code>module.require</code> method provides a way to load a module as if
<code>require()</code> was called from the original module.

</p>
<p>Note that in order to do this, you must get a reference to the <code>module</code>
object.  Since <code>require()</code> returns the <code>module.exports</code>, and the <code>module</code> is
typically <em>only</em> available within a specific module&#39;s code, it must be
explicitly exported in order to be used.

</p>

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